Abstract: Abstract: The two-stage coproduction of hydrogen and methane using cellulosic biomass, such as reed straw, is a promising technology for achieving energy saving and emission reduction and developing a circular economy. The enhancement of hydrogen and methane coproduction from reed straw under enzyme pretreatment was evaluated during anaerobic fermentation. The effects of cellulase pretreatment on biogas production performance and intermediate metabolites' characteristics were investigated in this study. In addition, the combination of polymerase chain reaction (PCR) amplification of 16S rRNA genes with denaturing gradient gel electrophoresis (DGGE) analysis was used to study the composition and succession of bacterial community in fermentative biohydrogen with methanogenesis integration system. The results showed that the maximum accumulative biogas production and hydrogen proportion were 42.5 mL/g and 52.1% respectively in hydrogenogenic stage. And the maximum accumulative biogas production of 137.5 mL/g was 5.36 times higher than the control in methane prodution stage. However, the highest methane proportion of 68.4% in control test was similar to those under cellulase pretreatment. Therefore, the cellulase pretreatment has the benefit of structural damage on refractory organics while improving the hydrogen production potential in this study. Usually, hydrogen and methane formation is accompanied by volatile fatty acids (VFAs) generation during anaerobic digestion process. Hence, the composition and concentration of soluble metabolites produced were useful indicators for monitoring the hydrogenogenic process. The investigation of the soluble metabolites at the end of each stage showed that the main VFAs were distributed under cellulase pretreatment compared with the control. The composition of VFAs in hydrogenogenic stage was butyric acid and acetic acid, indicating that butyric-acid type fermentation was established. During the methanogenic stage, the butyric acid was consumed and the propionic and valeric acid were produced more. These results showed that cellulase pretreatment might be attributed to the diversity of microbial populations in 2 stages after enrichment. The sequences of 16S rDNA DGGE predominant band fragments were determined by comparison with NCBI database. The DGGE patterns showed that the 2 stages experienced different microbial community structure changes during the period. Early in hydrogenogenic stage, the low similarity of bacterial communities was observed. And then the high similarity with Deiss coefficient of 83.6% (lane: H3, H4) and 87.4% (lane: M6, M7) was obtained in peak production period of hydrogen and methane, respectively. The Cs value was reduced to 51.5 at the end of methane production stage (lane: M6, M9). The majority of the sequences obtained were affiliated with Clostridium thermocellum (Band 20), Enterobacter aerogenes (Band 28) and Sedimentibacter (Band 29). In hydrogenogenic stage, the dominant microorganism was Enterobacter aerogenes (Band 28), which can produce hydrogen and dramatically enhance the hydrogen production performance. A hydrogen-producing acetogenic bacterium of Sedimentibacter (Band 29) was also the dominant bacterium, which can produce hydrogen and acetic acid in hydrogen and methane coproduction process. The dominant microorganism of Clostridium thermocellum (Band 20) existed in 2 stages, which can degrade cellulose and play an important role in reed straw utilization process. Hence, the maximum cumulative biogas yield and proportion were increased dramatically under the cellulase pretreatment, which directly impacted the hydrogen and methane production ability. The result provides an important microbiology theoretical basis for the biofortification in biogas coproduction process from cellulosic biomass.